1* Overview
2 3 Mass Storage Gadget (or MSG) acts as a USB Mass Storage device,
4 appearing to the host as a disk or a CD-ROM drive. It supports
5 multiple logical units (LUNs). Backing storage for each LUN is
6 provided by a regular file or a block device, access can be limited
7 to read-only, and gadget can indicate that it is removable and/or
8 CD-ROM (the latter implies read-only access).
9 10 Its requirements are modest; only a bulk-in and a bulk-out endpoint
11 are needed. The memory requirement amounts to two 16K buffers.
12 Support is included for full-speed, high-speed and SuperSpeed
13 operation.
14 15 Note that the driver is slightly non-portable in that it assumes
16 a single memory/DMA buffer will be useable for bulk-in and bulk-out
17 endpoints. With most device controllers this is not an issue, but
18 there may be some with hardware restrictions that prevent a buffer
19 from being used by more than one endpoint.
20 21 This document describes how to use the gadget from user space, its
22 relation to mass storage function (or MSF) and different gadgets
23 using it, and how it differs from File Storage Gadget (or FSG)
24 (which is no longer included in Linux). It will talk only briefly
25 about how to use MSF within composite gadgets.
26 27* Module parameters
28 29 The mass storage gadget accepts the following mass storage specific
30 module parameters:
31 32 - file=filename[,filename...]
33 34 This parameter lists paths to files or block devices used for
35 backing storage for each logical unit. There may be at most
36 FSG_MAX_LUNS (8) LUNs set. If more files are specified, they will
37 be silently ignored. See also “luns” parameter.
38 39 *BEWARE* that if a file is used as a backing storage, it may not
40 be modified by any other process. This is because the host
41 assumes the data does not change without its knowledge. It may be
42 read, but (if the logical unit is writable) due to buffering on
43 the host side, the contents are not well defined.
44 45 The size of the logical unit will be rounded down to a full
46 logical block. The logical block size is 2048 bytes for LUNs
47 simulating CD-ROM, block size of the device if the backing file is
48 a block device, or 512 bytes otherwise.
49 50 - removable=b[,b...]
51 52 This parameter specifies whether each logical unit should be
53 removable. “b” here is either “y”, “Y” or “1” for true or “n”,
54 “N” or “0” for false.
55 56 If this option is set for a logical unit, gadget will accept an
57 “eject” SCSI request (Start/Stop Unit). When it is sent, the
58 backing file will be closed to simulate ejection and the logical
59 unit will not be mountable by the host until a new backing file is
60 specified by userspace on the device (see “sysfs entries”
61 section).
62 63 If a logical unit is not removable (the default), a backing file
64 must be specified for it with the “file” parameter as the module
65 is loaded. The same applies if the module is built in, no
66 exceptions.
67 68 The default value of the flag is false, *HOWEVER* it used to be
69 true. This has been changed to better match File Storage Gadget
70 and because it seems like a saner default after all. Thus to
71 maintain compatibility with older kernels, it's best to specify
72 the default values. Also, if one relied on old default, explicit
73 “n” needs to be specified now.
74 75 Note that “removable” means the logical unit's media can be
76 ejected or removed (as is true for a CD-ROM drive or a card
77 reader). It does *not* mean that the entire gadget can be
78 unplugged from the host; the proper term for that is
79 “hot-unpluggable”.
80 81 - cdrom=b[,b...]
82 83 This parameter specifies whether each logical unit should simulate
84 CD-ROM. The default is false.
85 86 - ro=b[,b...]
87 88 This parameter specifies whether each logical unit should be
89 reported as read only. This will prevent host from modifying the
90 backing files.
91 92 Note that if this flag for given logical unit is false but the
93 backing file could not be opened in read/write mode, the gadget
94 will fall back to read only mode anyway.
95 96 The default value for non-CD-ROM logical units is false; for
97 logical units simulating CD-ROM it is forced to true.
98 99 - nofua=b[,b...]
100 101 This parameter specifies whether FUA flag should be ignored in SCSI
102 Write10 and Write12 commands sent to given logical units.
103 104 MS Windows mounts removable storage in “Removal optimised mode” by
105 default. All the writes to the media are synchronous, which is
106 achieved by setting the FUA (Force Unit Access) bit in SCSI
107 Write(10,12) commands. This forces each write to wait until the
108 data has actually been written out and prevents I/O requests
109 aggregation in block layer dramatically decreasing performance.
110 111 Note that this may mean that if the device is powered from USB and
112 the user unplugs the device without unmounting it first (which at
113 least some Windows users do), the data may be lost.
114 115 The default value is false.
116 117 - luns=N
118 119 This parameter specifies number of logical units the gadget will
120 have. It is limited by FSG_MAX_LUNS (8) and higher value will be
121 capped.
122 123 If this parameter is provided, and the number of files specified
124 in “file” argument is greater then the value of “luns”, all excess
125 files will be ignored.
126 127 If this parameter is not present, the number of logical units will
128 be deduced from the number of files specified in the “file”
129 parameter. If the file parameter is missing as well, one is
130 assumed.
131 132 - stall=b
133 134 Specifies whether the gadget is allowed to halt bulk endpoints.
135 The default is determined according to the type of USB device
136 controller, but usually true.
137 138 In addition to the above, the gadget also accepts the following
139 parameters defined by the composite framework (they are common to
140 all composite gadgets so just a quick listing):
141 142 - idVendor -- USB Vendor ID (16 bit integer)
143 - idProduct -- USB Product ID (16 bit integer)
144 - bcdDevice -- USB Device version (BCD) (16 bit integer)
145 - iManufacturer -- USB Manufacturer string (string)
146 - iProduct -- USB Product string (string)
147 - iSerialNumber -- SerialNumber string (sting)
148 149* sysfs entries
150 151 For each logical unit, the gadget creates a directory in the sysfs
152 hierarchy. Inside of it the following three files are created:
153 154 - file
155 156 When read it returns the path to the backing file for the given
157 logical unit. If there is no backing file (possible only if the
158 logical unit is removable), the content is empty.
159 160 When written into, it changes the backing file for given logical
161 unit. This change can be performed even if given logical unit is
162 not specified as removable (but that may look strange to the
163 host). It may fail, however, if host disallowed medium removal
164 with the Prevent-Allow Medium Removal SCSI command.
165 166 - ro
167 168 Reflects the state of ro flag for the given logical unit. It can
169 be read any time, and written to when there is no backing file
170 open for given logical unit.
171 172 - nofua
173 174 Reflects the state of nofua flag for given logical unit. It can
175 be read and written.
176 177 Other then those, as usual, the values of module parameters can be
178 read from /sys/module/g_mass_storage/parameters/* files.
179 180* Other gadgets using mass storage function
181 182 The Mass Storage Gadget uses the Mass Storage Function to handle
183 mass storage protocol. As a composite function, MSF may be used by
184 other gadgets as well (eg. g_multi and acm_ms).
185 186 All of the information in previous sections are valid for other
187 gadgets using MSF, except that support for mass storage related
188 module parameters may be missing, or the parameters may have
189 a prefix. To figure out whether any of this is true one needs to
190 consult the gadget's documentation or its source code.
191 192 For examples of how to include mass storage function in gadgets, one
193 may take a look at mass_storage.c, acm_ms.c and multi.c (sorted by
194 complexity).
195 196* Relation to file storage gadget
197 198 The Mass Storage Function and thus the Mass Storage Gadget has been
199 based on the File Storage Gadget. The difference between the two is
200 that MSG is a composite gadget (ie. uses the composite framework)
201 while file storage gadget was a traditional gadget. From userspace
202 point of view this distinction does not really matter, but from
203 kernel hacker's point of view, this means that (i) MSG does not
204 duplicate code needed for handling basic USB protocol commands and
205 (ii) MSF can be used in any other composite gadget.
206 207 Because of that, File Storage Gadget has been removed in Linux 3.8.
208 All users need to transition to the Mass Storage Gadget. The two
209 gadgets behave mostly the same from the outside except:
210 211 1. In FSG the “removable” and “cdrom” module parameters set the flag
212 for all logical units whereas in MSG they accept a list of y/n
213 values for each logical unit. If one uses only a single logical
214 unit this does not matter, but if there are more, the y/n value
215 needs to be repeated for each logical unit.
216 217 2. FSG's “serial”, “vendor”, “product” and “release” module
218 parameters are handled in MSG by the composite layer's parameters
219 named respectively: “iSerialnumber”, “idVendor”, “idProduct” and
220 “bcdDevice”.
221 222 3. MSG does not support FSG's test mode, thus “transport”,
223 “protocol” and “buflen” FSG's module parameters are not
224 supported. MSG always uses SCSI protocol with bulk only
225 transport mode and 16 KiB buffers.
226